How to count the number of keys matching a pattern?

Question

How can I find the count of all the keys that has a matching pattern.

For example, there are two keys abc:random-text-1 and abc:random-text-2 .

Answer 1

From here:

eval "return #redis.pcall('keys', 'abc:*')" 0

It's not O(1), but at least the count is done on the server side.